Moistening apparatus and method for duplicator sheets

ABSTRACT

A liquid absorbent applicator for applying a liquid to the copy sheets of a duplicating machine, is maintained at a constant moisture content. Two sensing electrodes engage the applicator and are connected with control means for a motor driving a supply means for the liquid, such as a pump or liquid elevator roller. When high resistance between the electrodes, and consequently low current between the same, indicates that the applicator is too dry, the control means actuate the electric motor which drives the pump or elevator roller to increase the moisture content of the applicator to a desired constant value. When the moisture content is too high, the control means respond to the sensing means to stop the motor and the supply of moistening liquid.

[4 Dec. 4, 1973 [22] Filed:

[ 1 MOHSTENING APPARATUS AND METHOD FOR DUPLICATOR SHEETS [76] Inventor:Gerhard Ritzerfeld, Schloremer Allee 14, 1000 Berlin 33, Germany Jan.17, 1972 21 Appl. No.: 218,380

[30] Foreign Application Priority Data Jan. 18, 1971 Germany; P 21 03495.7 Nov. 13, 1971 Germany P 21 57 225.8

[52] 1.1.5. C1 101/147, ll/132.5, 101/148,

[51] Int. Cl. B411 23/12, B41f 31/02, B050 1/06 [58] Field'of Search101/132.5, 147, 148, 101/348, 349, 350; 118/264, 265, 266, 268; 318/483[56] References Cited 3,195,816 7/1965 Mercer 318/483 3,499,383 3/1970Southam... 101/148 3,555,289 l/197l Sobkow 318/483 PrimaryExaminer-Robert E. Pulfrey Assistant ExaminerPaul T. SewellAttorney-Michael S. Striker [57] ABSTRACT A liquid absorbent applicatorfor applying a liquid to the copy sheets of a duplicating machine, ismaintained at a constant moisture content. Two sensing electrodes engagethe applicator and are connected with control means for a motor drivinga supply means for the liquid, such as a pump or liquid elevator roller.When high resistance between the electrodes, and consequently lowcurrent between the same, indicates that the applicator is too dry, thecontrol means actuate the electric motor which drives the pump orelevator roller to increase the moisture content of the applicator to adesired constant value. When the moisture content is too high, thecontrol means respond to the sensing means to stop the motor and thesupply of moistening liquid.

SHEET 2 BF 5 PATENTED DEC 4 I975 Fig.3

MOISTENING APPARATUS AND METHOD'FOR DUPLICATOR SHEETS BACKGROUND OF THEINVENTION The present invention relates to amethod and apparatus formoistening the transported copy sheets of a rotary duplicator in whichmoist copy sheets are pressed against a master sheet. In known rotaryduplicating machines, a pump, or a valve, controls the amount of liquidflowing to the felt pad-by which the copy sheets are moistened. Thevalve or the pump has to be manually operated or periodically actuatedunder the control of a rotary cam. The actualamount of supplied liquidis either uncontrolled; or aconstant amount of liquid is supplied to theabsorbent moistening pad. In both ways of operation; the actual moisturecontent of the. absorbent pad is not considered. If long copy sheetshave to be moistened in rapid succession,

a greater amount of liquid is taken from the absorbent moistening pad,then when the copy sheets are narrow cards or slips are moistened. Thisis particularly the case in rotary duplicators which print-from a mastersheet containing the entire text, selected lines on some copy sheets,and a series of lines on another copy sheet which are accordingly shortor long. Some of the copy sheets have full length, other half length andother quarter length, each type of copy sheets being supplied indifferent numbers, and may also have different paper qualities,absorbing different amounts of liquid when moistened by the applicatorfelt pad.

In the arrangement of the prior art; the applicator pads may contain aninsufficient amount of liquid, and insufficiently moisten the respectivecopy sheets,- or sometimes have too' much liquid absorbed. In' the firstcase, more liquid has to be supplied by manual operation, and in thesecond casea valve had to be operated to reduce the amount of liquidsupplied tothe applicator' pad. Under practical conditions, frequentlythe copy sheets are moistened too much, resulting in the transfer of atoo great amount of duplicator ink from the master sheet, and apremature wearing offzof the mirror-reversed characters of the mastersheet.

Furthermore, since rotary duplicators use an alcoholic liquid asamoistening liquid; the moisture'cont'ent of the applicator felt padalso depends on the temperature of the room in which the machineoperatessince at higher temperatures, the alcohol evaporates'faster than at lowtemperatures.

SUMMARY OF THE INVENTION It is one object of the inventionto overcomethe above-explained disadvantages of the prior art, and to eff'ectauniform moistening of all'copy sheets supplied to the duplicator,irrespective of the size and number of the copy sheets, and of theprevailing temperature.

Another object of the invention isto maintain the moisture content ofliquid absorbent-applicator. means constant.

Another object of the invention is to'obtain-uniforrnly clear printedtexts on successive copy sheets by properly moistening each copy sheet.

Another object of the invention is to regulate. the amount of liquidsupplied. to absorbent applicator means in accordance with the moisturecontent of the absorbent applicator determined by sensing the mois-'ture content.

With these objects in view, the moistening method of the inventioncomprises electrically sensing the moisture content of a liquidabsorbent pad when the pad transfers absorbed liquid to duplicator copysheets; and automatically supplying under the control of the sensingmeans, a replenishing liquid to the pad when the sensed moisturecontent-is insufficient. In this manner, a substantially constantmoisture content is maintained in the absorbent pads. 1

An apparatus according to the invention comprises applicator meansincluding a, liquid absorbent means, such as a felt pad; sensing means,preferably including two sensing electrodes for sensing the moisturecontent of the absorbent means; motor means; control means controlled bythe sensing means; and supply means such as a pump, or an elevatorroller partly immersed in a liquid, driven by the motor-means forsupplying liquid to the absorbent means.

The absorbent means supplies the moistening liquid tosuccessiveiiiiplicator copy sheetsso that the moisture content of theabsorbent means is decreased bytransfer of the liquid to the sheets.

When the sensing electrodes sense a low moisture content of theabsorbent means, which would be insufficient for properly moistening thecopy sheets, the control means respond to the sensed moisture content tostart the motor means so that the supply means, for example the motordriven pump, supplies the liquid to the absorbent means. Consequently,the sensed moisture content cannot decrease below a predeterminedminimum, and on the other hand, when the moisture content becomes toohigh, which may cause dripping, the control means respond to a signalfrom the sensing means to stop the motor and thereby the liquid supplymeans.

Iii the circuit of the motor and of the electrodes, electronic controlmeans are advantageously provided for controlling the'motor of thesupply pump depending on the amount of current flowing through theabsorbent means.

In a modified embodiment of the invention, a container contains amoistening liquid at a constant level, andan elevator roller, partlyimmersed in the liquid, is drivenby the motor to raise the liquid to thelevel of a transfer roller which transfers the liquid to an absorbentapplicator, pad.

The applicator pad either directly cooperates with the duplicator copysheets, or transfers liquid to a transporting roller by which theduplicator copy sheets are transported sothat liquid is transferred bythe transporting roller to the duplicator copy sheets.

It is advantageous to provide in the circuit, a potentiometer orvariable resistance for the minimum current and maximum current flowingthrough the motor. The applicator pad is advantageotisly made of anabsorbentfelt consisting of wool fibers.

The novel features which are considered as characteristic for'theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from thefollowing description of specific embodimentswhen read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation, partiallyin section, illustrating a rotary duplicator with one side wall omitted,and including a diagrammatic illustration of the circuit of theapparatus of the invention;

FIG. 2 is a cross-sectional view illustrating applicator means inaccordance with one embodiment of the invention;

FIG. 3 is an enlarged sectional view taken on line C-D in FIG. 2;

FIG. 4 is a fragmentary cross-sectional view illustrating the drivingconnection between the motor and the pump in'the embodiment of FIG. 1;

FIG. 5 is a sectional view taken on line E-F in FIG.

FIG. 6 is a sectional view corresponding to FIG..2, but illustrating theapplicator means cooperating with the transporting roller for duplicatorcopy sheets;

FIG. 7 is a side elevation, partially in section, illustrating aduplicating machine provided with a second embodiment of the invention,with one side wall omitted, and including a diagrammatic illustration ofthe circuit of the apparatus according to the invention;

FIG. 8 is a sectional view illustrating a detail of the embodiment ofthe invention shown in FIG. 7 on an enlarged scale; and

FIG. 9 is an enlarged sectional view illustrating a portion of FIG. 9,and including a diagrammatic illustration of the circuit of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1, aduplicator drum 1 is rotated on a shaft 1a by a motor, not shown,through a one revolution clutch, not shown, in the usual manner,

and is connected by a gear train 2, 3, 4 with a shaft 30 carrying cams5, 6, 7 which can be shifted in axial direction by lever means 39, 40,41 and 42, and which cooperate with a stepping mechanism including pawls37 and 38 and activators 34, 35, 36 so that cams 8', 9 and. 10cooperating with text lines are circumferentially displaced so that bymeans of the printing linkage 11-15, the counter pressure roller 16 ispressed against the duplicator drum 1 and the master sheet 17 forimprinting moistened copy sheets 18 supplied to the printing plane AB bya pair of transporting rollers 55, 64 after being moistened with analcoholic liquid by the applicator pad 21 of a moistening device 20.Duplicator machines of this type are known and described, for example,in my US. Pat. Nos. 2,868,116 and 3,283,701. The moistening device 20with the applicator pad 21 is shown in FIG. 1 in a position turned'by alinkage 20a to a position in contact with the top surface of aduplicator sheet 18 for moistening the same while it is transported bythe transporting rollers 55, 64 1 towards the printing plane AB.

As shown in FIGS. 1, 2 and 3, the flow of the liquid to the absorbentapplicator pad 21, which is held in an insulating container 33, issupplied by a supply pump driven by a motor 22. Pump 23 sucks through aninlet conduit 24, the moistening liquid from a reservoir or container,not shown, and discharges the liquid through the outlet conduit 25toaspray pipe 26 which has radial holes through which the liquid issprayed in container portion 33a toward the absorbent applicator pad 21,as best seen in FIG. 2. The absorbent applicator pad 21 preferablyconsists of a felt made of wool fibers.-

printing plane AB.

In order to measure themoisture-content of the absorbent applicator pad21, sensing means including spaced electrodes 27, 28 are used, which areinsulated by a non-conductive wall 43 from the conductive parts of thedevice 20. Insulating sleeves 44 and 45 envelop the connector wiresleading to the electrodes 27 and 28. A switch 31 is connected with asource of ,voltage, as shown in FIG. 3, and also connected to the motor22, and to an electronic control means 29. An adjustable potentiometer32 is connected in series with the other sensing electrode 28. Thevoltage is transformed in control means 29 to a lower voltage requiredfor the circuit 27, 28, 29, 32.

A current flows between electrodes 27, 28 through the respective part ofthe absorbent applicator pad 21, and also through control means '29, andthe amount of current flowing between the electrodes 27 and 28 isdetermined by the resistance of the respective applicator pad portionwhich depends on the moisture content of the applicator pad 21. If theabsorbent applicator pad 21 is heavily saturated with the liquid, theresistance between the electrodes-27, 28 is low, and a strong currentwill flow to control means 29 which stops motor 22 when the currentreaches a maximum. If additional duplicator sheets 18 are transported tothe moistening device, and the absorbent applicator pads 22 transfersuch an amount of liquid to the duplicator sheets that the pad becomestoo dry, the resistance between the electrodes 27 and 28 is increased,and a smaller current flows through the control means 29.

When a predetermined minimum current is reached, the electronic controlmeans 29 generates a starting pulse for motor 22 which then operatespump 23 to moisten the absorbent applicator 21 with the liquid until thepad 21 is again sufi'lciently saturated. If the applicator pad becomestoo moist, the electronic control means 29 generates a pulse by whichthe motor 22 is disconnected.

Switch 31 is operated when the main switch of the duplicating machine isactuated, and starts motor 22 with pump 23 if the applicator pad 21 isdried out so that even after a prolonged standstill of the duplicatingmachine, the applicator pad 21 is applied with the required amount ofliquid so that even the first fed duplicator sheet 18 is properlymoistened. The potentiometer 32 is manually operated, and adds itsvariable re- I sistance to the resistance between the electrodes 27 and28. In this 'manner, the amount of liquid supplied to the absorbentapplicator pad 21 can be regulated by varyingthe resistance ofpotentiometer 32 in such a manner that motor 22 starts at differentselected resistances so that an individual amount of liquid, dependenton different absorbent properties of the paper of which the sheet 18 aremade, can be supplied to the applicator pad 21.

As shown in FIG. 2, a container 33, and a closure 33a form a space inwhich the applicator pad 21 is held. The spray tube 26, supplied withliquid from pump 23, sprays liquid in the closed space against'theabsorbent applicator pad 21 on which the electrodes 27, 28 abut,

insulated from the closure part 33a by the wall of insulating material43.

FIGS. 4 and 5 illustrate a driving connection between the motor 22 andthe pump 23 which is schematically shown in FIG. 1. Pump 23 is shown inFIG. 4 to be a membrane pump. On the motor shaft .46, an-angular flange47 is secured as best seen in FIG. 5, and has a portion with a slot 48-in which a slide M11249, is mounted for adjustment, and is articulatedto a link 50 whose end is pivotally connected'with a member51 guided ina bore 56 of the pump housing 52 of the membrane pump 23.

When motor shaft 46 is rotated, member 51:is reciprocated so thatinchamber 54 alternately low=pressure and high pressure prevails so thatby meansof conventional valves, not shown, the liquid can be suckedthrough conduit 24and dischargedfrom conduit. 25 into the spraying tube26. By adjusting the position of bolt and nut means 49 in slot 48,thezstroke of .member- 51 and of the'membrane 53'can be increased ordecreased so that the amount of liquid transported by the pump 23 can beadjusted.

FIG. 6 illustrates an applicatordevice which fully corresponds to theapplicator device20 described .with reference to FIGS. 1 to 3, buttheapplicator'pad 21 is not in contact with the duplicator copy. sheet18:38 shown in FIG. 1, but in sliding contactwithxthe rotarytransporting roller 55 which cooperates with alarger transporting roller64 to transport the duplicatorsheets 18. Evidently, liquid from theapplicator; pad.2l;-is. transferred by transporting roller 5S=tothe-surface of the. duplicator sheets transported by transportingrollers 55, 64.

The embodiment of the invention shown in FIG. 7 is provided in aduplicating machineas described. with reference to FIG. 1, andconsequently, correspondingreference numerals are used in FIGS. l-to 7forlike' parts. The moistening device includes an absorbentapplicatorpad 21 cooperating with transporting roller 23- which corresponds toroller 23 in FIG. 6..Applicator pad 21 is held between insulating walls43, 43a secured to container 33. A closure 33b closes container 33 whichis filled with a liquid to a predetermined level maintained by overflowopenings, as shown in FIG. 7.

At the beginning of a duplicating operation, the moistening device 20'is turned to a position in which applicator pad 21 is in contact-withzthes transfer and transporting roller 23while the moisteningdevice 205 is tilted rearward with reference to the free end of theapplicator pad'21, so that no drops of the-supplied liquid can fall ontothe transfer roller23 during the moist-. ening operation, as requiredfor theproductionof a clearimprint. An excess'amountof liquidin theappli.--

cator pad 21 may flow through. the openings, not shown, in the adjacentside wall into container'33.

The applicator pad 21 is held between thenonconductive insulating partsor. walls 43, 43a. Aspring 62 presses insulating part 43 and applicatorpad 21 against transporting roller 23, while the inner end. of theapplicatorpad 21 is urged by spring6llagainst a,

that liquid is transferred from the elevator roller 26 to the transferroller 60'and from the same to the applicator pad 21, as best seen inFIG. 8. Transporting roller 23 further transfers liquid from theapplicator pad 21- so that a duplicator sheet 18 transported betweenrollers 23 and 64 is moistened. Consequently,.roller 23 has twofunctions, namely moistening the copy sheet 18, and transporting thecopy sheet 18 to the printing line AB of the duplicator.

Excessive moistening of the applicator-pad21 is prevented since motor 22drives the elevator roller 26 only if the applicator pad 21-is so dry,and has such a low moisture content, that a clear imprint on theduplicator sheet 18 by the master sheet 17 is no longer assured.

As in the embodiment of FIGS. 1 to 6, the insulated electrodes 27 and 28engage spaced portion of the applicator-pad 21. As shown in FIG. 9, aswitch 31 is connected in series with a source of voltage and withcontrol means 29, which is connected with electrode 28 by potentiometer32 and controls motor 22.

Dependent on the moisture content of the applicator pad 21 sensed by thesensing electrodes 27 and 28, different currents How to the controlmeans 29 which includes an amplifier for supplying current for drivemotor-22. If .the applicator pad 21 is highly saturated with liquid, theresistance between electrodes 27 and 28 is low, and a strong currentflows which effects a disconnection of 7 motor 22 by the electroniccontrol means-29. If more duplicator sheets 18 are fed and moistened,the applicator pad 21 becomes dry, the moisture content is reduced, andthe resistance between sensing electrodes 27, 28 is increased so that asmaller current flows.

When a predetermined minimum current is obtained, the control means 29generates a starting impulse for motor 22 which then drives throughshaft 63 the elevator roller 26 so that liquid is supplied throughtransfer roller 60 to the applicator pad 21 until the same. issufficiently saturated, and the sensing electrodes 27,28 sense a maximumcurrent and cause control means 29 to stop motor 22.

Switch 31 is operated at the same time as the main switch of theduplicator, and starts motor 23 with elevator roller 26 when theapplicator pad 21 is dried out, so that even after a prolongedstandstill of the duplicating machine, the applicator pad. 21 isproperly moistened, so that even the first duplicator sheet 18 issufficiently moistened for obtaining a clear imprint from the mastersheet 17.

The manually operated potentiometer 32 regulates the flow of liquidsupplied to the absorbent applicator pad.21- varyingthe predeterminedresistance between the electrodes 27 and 28, so that motor 22 alsostarts at different resistances between electrodes 27, 28 wherebyindividual amounts of liquid can be transferred to the duplicator sheets18, depending on differentproperties of the used duplicator sheets 18.

From the above ;description it will become apparent that in theembodimentof FIGS. 1,-6, theipump 23 and in the embodiment of FIGS. 7-9,the elevator roller 26 and the transfer roller 60, constitute supplymeans driven from motor 22 to supply liquid to ,the liquid absorbentapplicator pad 21 of the applicator means, which may either directlymoisten the duplicator sheets 18, or moisten the transporter roller 55which transfers the liquid to the duplicator sheets. Inall embodimentsof the invention, the speed of the motor 23 and of the respective supplymeans 23, 25, 26 or 26a, 60, depends on the resistance of the portion ofthe applicator pad 21 between the sensing electrode 27 and 28, theresistance being dependent on the moisture content of the absorbentapplicator pad 21.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofmoistening apparatus for duplicator sheets difi'ering from the typesdescribed above.

While the invention has been illustrated and described as embodied in aliquid supply means for an absorbent pad driven by a motor operatingonly if the pad becomes too dry, it is not intended to be limited to thedetails shown, since various modifications and structural changes may bemade without departing in any way from the spirit of the presentinvention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art fairly constitute essentialcharacteristics of tye generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

I claim:

1. Moistening apparatus for duplicator sheets, comprising, incombination, applicator means including a liquid absorbent means, andadapted to apply a moistening liquid to successive duplicator sheets sothat the moisture content of said absorbent means is decreased bytransfer of said liquid to the sheets; sensing means for sensing themoisture content of said absorbent means; motor means; control meanscontrolled by said sensing means to start and stop said motor meansdepending on the sensed moisture content of said absorbent means; andmovable supply means for said liquid driven by said motor means forsupplying said liquid to said absorbent means when the sensed moisturecontent decreases below a predetermined minimum so that the sheets aremaintained at a substantially constant moisture content.

2. Duplicator moistening apparatus for moistening duplicator copy sheetscomprising, in combination, applicator means including a liquidabsorbent means, and adapted to apply a moistening liquid contained insaid absorbent means to successive duplicator sheets so that themoisture content of said absorbent means is decreased-by transfer ofsaid liquid to said duplicator copy sheets, said absorbent means havingan electric resistance dependent on the moisture content thereof;sensing means including two spaced electrodes in contact with saidabsorbent means for sensing the resistance of the portion of saidabsorbent means for sensing the resistance of the portion of saidabsorbent means between said two spaced electrodes; motor means; controlmeans controlled by said sensing means depending on the resistance andthe moisture content of said portion of said absorbent means to startand stop said motor means when said moisture content is a predeterminedminimum and maximum, respectively; and movable supply means for saidliquid driven by said motor means to supply said liquid to saidabsorbent means when the sensed moisture content decreases below saidpredetermined minimum, and to stop with said motor meanswhenthe moisturecontent is increased to said predetermined maximum whereby said copysheets are substantially uniformly moistened.

3. Apparatus as claimed in claim 2, wherein said control means are'electronic control means; and wherein said motor rrieans includes anelectric motor connected with and started and stopped, respectively, bysaid electronic control means when the current flowing between saidelectrodes is a predetermined minimum and maximum current, respectively,due to reduced and increased moisture content, respectively, andcorrespondingly increased and reduced resistance, respectively, of saidportion of said absorbent means.

4. Apparatus as claimed in claim 2, wherein said supply means include ameans-for spraying the moistening liquid onto said absorbent means; anda pump driven by said motor means for pumping said liquid to-saidspraying means.

5. Apparatus as claimed in claim 2 wherein said applicator means includeinsulating container means supporting said absorbent means and saidsensing means; and wherein said supply means are at least partly locatedin said container means for supplying liquid to said absorbent means.

6. Apparatus as claimed in claim 2 wherein said absorbent means includesan absorbent pad in contact with transported sheets; and comprisingtransporting roller means for transporting the sheets.

7. Apparatus as claimed in claim 2 wherein said applicator means includea transporting roller for transporting the sheets while rolling on thesurface of the sheets, and wherein said absorbent means include anabsorbent pad slidingly abutting said transporting roller so that saidtransporting roller is moistened by said liquid and transfers saidliquid to the sheets.

8. Apparatus as claimed in claim 2 wherein said supply means include apump driven by said motor means for pumping said moistening liquid ontosaid absorbent means when the sensed moisture content is insufficient.

9. Apparatus as claimed in claim 2 wherein said supply means include acontainer for said moistening liquid, an elevator roller partly immersedin the liquid in said container, and a transfer roller in rollingcontact with said elevator roller and in sliding contact with saidabsorbent means; and wherein said motor drives said elevator roller whenthe sensed moisture content of the absorbent means is insufficient.

10. Apparatus as claimed in claim 9 wherein said container is closed andenvelopes said elevator roller,

' said transfer roller, and the part of said absorbent 7 means engagedby'said transfer roller, and includes means for maintaining apre-determined highest level of said moistening liquid in said containerso that said absorbent means and said transfer roller are located abovesaid constant level.

11. Apparatus. as claimed in claim 2 wherein said motor means includesan electric motor, and circuit means connecting said motor with saidcontrol means; and a variable resistor in said circuit means foradjusting the maximum and minimum currents supplied to said controlmeans for stopping and starting said'motor.

12. Apparatus as claimed in claim 11 wherein said circuit means includea switch for stopping and starting said motor by disconnecting andconnecting, respectively, said motor from and to a source of voltage.

liquid to a roller rolling on duplicator sheets for moistening the same;and automatically supplying under the control of said sensing meansreplenishing liquid to said pad when the sensed moisture content isinsufficient, so as to maintain a substantially constant moisturecontent in said absorbent pad.

1. Moistening apparatus for duplicator sheets, comprising, incombination, applicator means including a liquid absorbent means, andadapted to apply a moistening liquid to successive duplicator sheets sothat the moisture content of said absorbent means is decreased bytransfer of said liquid to the sheets; sensing means for sensing themoisture content of said absorbent means; motor means; control meanscontrolled by said sensing means to start and stop said motor meansdepending on the sensed moisture content of said absorbent means; andmovable supply means for said liquid driven by said motor means forsupplying said liquid to said absorbent means when the sensed moisturecontent decreases below a predetermined minimum so thAt the sheets aremaintained at a substantially constant moisture content.
 2. Duplicatormoistening apparatus for moistening duplicator copy sheets comprising,in combination, applicator means including a liquid absorbent means, andadapted to apply a moistening liquid contained in said absorbent meansto successive duplicator sheets so that the moisture content of saidabsorbent means is decreased by transfer of said liquid to saidduplicator copy sheets, said absorbent means having an electricresistance dependent on the moisture content thereof; sensing meansincluding two spaced electrodes in contact with said absorbent means forsensing the resistance of the portion of said absorbent means forsensing the resistance of the portion of said absorbent means betweensaid two spaced electrodes; motor means; control means controlled bysaid sensing means depending on the resistance and the moisture contentof said portion of said absorbent means to start and stop said motormeans when said moisture content is a predetermined minimum and maximum,respectively; and movable supply means for said liquid driven by saidmotor means to supply said liquid to said absorbent means when thesensed moisture content decreases below said predetermined minimum, andto stop with said motor means when the moisture content is increased tosaid predetermined maximum whereby said copy sheets are substantiallyuniformly moistened.
 3. Apparatus as claimed in claim 2, wherein saidcontrol means are electronic control means; and wherein said motor meansincludes an electric motor connected with and started and stopped,respectively, by said electronic control means when the current flowingbetween said electrodes is a predetermined minimum and maximum current,respectively, due to reduced and increased moisture content,respectively, and correspondingly increased and reduced resistance,respectively, of said portion of said absorbent means.
 4. Apparatus asclaimed in claim 2, wherein said supply means include a means forspraying the moistening liquid onto said absorbent means; and a pumpdriven by said motor means for pumping said liquid to said sprayingmeans.
 5. Apparatus as claimed in claim 2 wherein said applicator meansinclude insulating container means supporting said absorbent means andsaid sensing means; and wherein said supply means are at least partlylocated in said container means for supplying liquid to said absorbentmeans.
 6. Apparatus as claimed in claim 2 wherein said absorbent meansincludes an absorbent pad in contact with transported sheets; andcomprising transporting roller means for transporting the sheets. 7.Apparatus as claimed in claim 2 wherein said applicator means include atransporting roller for transporting the sheets while rolling on thesurface of the sheets, and wherein said absorbent means include anabsorbent pad slidingly abutting said transporting roller so that saidtransporting roller is moistened by said liquid and transfers saidliquid to the sheets.
 8. Apparatus as claimed in claim 2 wherein saidsupply means include a pump driven by said motor means for pumping saidmoistening liquid onto said absorbent means when the sensed moisturecontent is insufficient.
 9. Apparatus as claimed in claim 2 wherein saidsupply means include a container for said moistening liquid, an elevatorroller partly immersed in the liquid in said container, and a transferroller in rolling contact with said elevator roller and in slidingcontact with said absorbent means; and wherein said motor drives saidelevator roller when the sensed moisture content of the absorbent meansis insufficient.
 10. Apparatus as claimed in claim 9 wherein saidcontainer is closed and envelopes said elevator roller, said transferroller, and the part of said absorbent means engaged by said transferroller, and includes means for maintaining a pre-determined highestlevel of said moistening liquid in said container so that said absorbentmeans and said transfer roller are located above said constant level.11. Apparatus as claimed in claim 2 wherein said motor means includes anelectric motor, and circuit means connecting said motor with saidcontrol means; and a variable resistor in said circuit means foradjusting the maximum and minimum currents supplied to said controlmeans for stopping and starting said motor.
 12. Apparatus as claimed inclaim 11 wherein said circuit means include a switch for stopping andstarting said motor by disconnecting and connecting, respectively, saidmotor from and to a source of voltage.
 13. Apparatus as claimed in claim2 wherein said absorbent means includes a pad consisting of fibrousfelt; and wherein said two spaced electrodes abut said felt pad, andcircuit means connecting said electrodes with said electric controlmeans.
 14. A moistening method for duplicator sheets comprisingelectrically sensing the moisture content of a liquid absorbent padwhile said pad transfers absorbed liquid to a roller rolling onduplicator sheets for moistening the same; and automatically supplyingunder the control of said sensing means replenishing liquid to said padwhen the sensed moisture content is insufficient, so as to maintain asubstantially constant moisture content in said absorbent pad.